The cellulose ester film is widely used for its optical and physical advantages as a protective film for a polarizing plate. However, the film is manufactured by the casting film formation method using a halogen-containing solvent, and therefore, costs required for solvent recovery impose a heavy burden on manufacturers. Various types of solvents other than the halogen-containing solvent have been tested, however, no alternative that could provide satisfactory solubility has ever been found out so far. In addition to the effort to find out an alternative solvent, efforts have been made to develop a new technique for dissolution such as a cooling method (for example, refer to Patent Document 1). However, further studies are necessary due to tough problems for industrial implementation. Further, a technique has been, disclosed to produce a cellulose ester film for optical use by a melt casting method without using a solvent (for example, refer to Patent Document 2).
There have been a disclosure of a technique wherein spectral properties and mechanical properties are improved by adding a hindered phenol antioxidant, a hindered amine light stabilizer or an acid scavenger into the cellulose ester at a predetermined ratio (for example, refer to Patent Document 3). In another technique having been disclosed, a phenol compound containing an acrylate group at the terminus is used to minimize reduction in the molecular weight of a polymer containing a cycloolefin structure and to obtain a polymer sheet characterized by excellent transparency and heat resistance (for example, refer to Patent Document 4).
However, the cellulose ester is a polymer characterized by a very high viscosity at the time of melting, as well as by a high glass transfer temperature. Even if the cellulose ester is melted to extrude from a die and to cast over a cooling drum or a cooling belt, it is difficult to achieve satisfactory leveling. The cellulose ester solidifies in a short time after extrusion. Thus, it has been found that such a cellulose ester may have a problem in forming a melt-cast film exhibiting a uniform optical property, for example, a uniform retardation property, specifically, a uniform retardation property in the lateral direction of the film. It has also been found that the cellulose ester film formed by a melt-casting method may be inferior to a cellulose ester film formed by a solution-casting method in physical properties, such as colaration of the film, haze, bright spots due to foreign substances (also referred to as bright foreign defects), since it is melted and cast at a higher temperature to form a film.
Patent Document 1 Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 10-95861
Patent Document 2 JP-A No. 2000-352620
Patent Document 3 JP-A No. 2003-192920
Patent Document 4 JP-A No. 2002-114949